Liquid crystal display devices are used not only typically for watches and calculators but also for various measuring instruments, automobile panels, word processors, electronic notebooks, printers, computers, televisions and others. Liquid crystal display devices utilize optical anisotropy and dielectric anisotropy of the liquid crystal material.
Typical liquid crystal display modes include a TN (twisted nematic) mode, an STN (super-twisted nematic) mode, a DS (dynamic scattering) mode, a GH (guest-host) mode, an FLC (ferroelectric liquid crystal) mode and the like; and as for the driving mode, multiplex driving has become more popular than conventional static driving, and further a simple matrix mode and recently an active matrix mode have become put into practical use.
In accordance with these display modes and driving modes, liquid crystal materials are required to have various properties, and a very large number of liquid crystalline compounds have been heretofore synthesized for satisfying those properties.
The properties required of liquid crystalline compounds may somewhat change according to display modes; however, a broad liquid crystal temperature range and stability to moisture, air, light, heat, electric field and the like are commonly required for any of those display modes.
At present, no single liquid crystalline compound capable of satisfying the requirements by itself is known, and some different types of liquid crystalline compounds are mixed or are further mixed with any other non-liquid crystalline compound for practical use. Mixing a plurality of compounds inevitably lowers the melting point or the like of the resulting mixture. Accordingly, liquid crystalline compounds capable of solely having a high phase transition temperature are desired in order that their mixture could still keep a practicable phase transition temperature even though the phase transition temperature are lowered by mixing them.
Liquid crystals can be divided into two major categories, one including thermotropic liquid crystals and the other including lyotropic liquid crystals. Of thermotropic liquid crystals, calamitic liquid crystals consisting of a rod-shaped molecule are being intensively studied in conjunction with electronic technology.
The calamitic liquid crystal phase includes a nematic liquid crystal phase, a smectic liquid crystal phase and a cholesteric liquid crystal phase. The cholesteric liquid crystal phase is a phase that appears when a nematic liquid crystal has an asymmetric factor or when a chiral additive (chiral dopant) is added to a nematic liquid crystal. In general, a liquid crystalline substance shows a phase change from a crystal or solid to a smectic phase and further to a nematic phase with elevation of the temperature thereof, and with further temperature elevation, it becomes an isotropic liquid. In the nematic liquid crystal phase, the molecules are aligned regularly in some degree, but there is no regularity with respect to the relative moleculargeometry. The individual molecules of nematic liquid crystals can freely move in the long axis direction thereof, and therefore have the advantage of low viscosity. The free energy of the nematic liquid crystal phase is the same irrespective of the alignment direction of the molecules, and therefore the molecular orientation can be changed in a given direction by an electric field or orientation treatment or the like given thereto, and accordingly, nematic liquid crystal molecules are widely used in liquid crystal displays and others.
Therefore, it is desirable that the lowermost limit of the temperature range within which a liquid crystal can have a nematic phase, namely, the temperature at which a liquid crystal changes from a crystal or solid state or a smectic phase to a nematic phase is low; and it is also desirable that the temperature at which a liquid crystal changes from a nematic phase to an isotropic liquid (nematic-isotropic transition temperature: TNI—generally this is referred to as a “clearing point”) (hereinafter, referred to as “N-I transition temperature”) is high and that the temperature range within which a liquid crystal shows a nematic phase is broad.
Some liquid crystalline compounds having a high clearing point have been heretofore reported. Patent Reference 1 describes a compound having a skeleton where a trans-1-sila-1,4-cyclohexylene or trans-4-sila-1,4-cyclohexylene group and a trans-1,4-cyclohexylene group linked to each other are bonded to a benzene ring via a carbonyloxy group, reporting its N-I transition temperature of from 50° C. to 171° C.
Patent Reference 2 shows that a tricyclic azine such as 1-(4-methylbenzylidene)-2-[4-(trans-4-propyl)cyclohexylbenzylidene]hydrazine or the like has an N-I transition temperature of from 227° C. to 265° C., reporting that, when mixed with a host liquid crystal having an N-I transition temperature of 116.7° C., the compound could elevate the N-I transition temperature of the mixture up to 144° C. to 157° C.
Further, Non-Patent Reference 1 reports that a 1-(4-cyanophenyl)-4-alkyl-substituted bicyclo[2,2,2]octane shows an N-I transition temperature of from 90 to 100° C.    Patent Reference 1: JP-A-8-119975    Patent Reference 2: JP-A-11-71338    Non-Patent Reference 1: G. W. Gray et al., J. Chem. Soc. Perkin II, 4765 (1981)